1. Field of the Invention
This invention relates to para-aramid pulp including meta-aramid fibrids for use as reinforcement material in products including for example friction materials, fluid sealing materials, and papers. The invention further relates to processes for making such pulp.
2. Description of Related Art
Fibrous and non fibrous reinforcement materials have been used for many years in friction products, sealing products and other plastic or rubber products. Such reinforcement materials typically must exhibit high wear and heat resistance.
Asbestos fibers have historically been used as reinforcement materials, but due to their health risks replacements have been made or proposed. However, many of these replacements do not perform as well as asbestos in one way or another.
Research Disclosure 74-75, published February 1980, discloses the manufacture of pulp made from fibrillated KEVLAR® brand para-aramid fibers of variable lengths and use of such pulp as a reinforcement material in various applications. This publication discloses that pulp made from KEVLAR® brand para-aramid fibers can be used in sheet products alone, or in combination with fibers of other materials, such as NOMEX® brand meta-aramid, wood pulp, cotton and other natural celluloses, rayon, polyester, polyolefin, nylon, polytetrafluoroethylene, asbestos and other minerals, fiberglass and other ceramics, steel and other metals, and carbon. The publication also discloses the use of pulp from KEVLAR® brand para-aramid fiber alone, or with KEVLAR® brand para-aramid short staple, in friction materials to replace a fraction of the asbestos volume, with the remainder of the asbestos volume being replaced by fillers or other fibers.
U.S. Pat. No. 5,811,042 (to Hoiness) discloses a composite friction or gasketing material made of a thermoset or thermoplastic matrix resin, fiber reinforcing material, and substantially fibril free aramid particles. Poly (p-phenylene terephthalamide) and poly(m-phenylene isophthalamide) are preferred fiber reinforcing materials, and the fibers can be in the form of floc or pulp.
U.S. Pat. No. 4,472,241 to Provost discloses a process for preparation of an aqueous suspension of aramid fibrids and aramid floc, suitable for the preparation of paper or pressboard. This disclosure teaches that a suspension of meta-aramid floc and meta-aramid fibrids can be refined together without damage to the meta-aramid floc. Meta-aramid floc, also, does not fibrillate and does not form a pulp like para-aramid floc does when refined.
U.S. Patent Application Publication 2003/0022961 (to Kusaka et al.) discloses friction materials made from a friction modifier, a binder and a fibrous reinforcement made of a mixture of (a) a dry aramid pulp and (b) wet aramid pulp, wood pulp or acrylic pulp. Dry aramid pulp is defined as an aramid pulp obtained by “the dry fibrillation method”. The dry fibrillation method is dry milling the aramid fibers between a rotary cutter and a screen to prepare the pulp. Wet aramid pulp is defined as an aramid pulp obtained by “the wet fibrillation method”. The wet fibrillation method is milling short aramid fibers in water between two rotary discs to form fibrillated fibers and then dehydrating the fibrillated fibers, i.e., the pulp. Kusaka et al further disclose a method of mix-fibrillating fibers by first mixing plural types of organic fibers that fibrillate at a definite ratio, and then fibrillating the mixture to produce a pulp.
There is an ongoing need to provide alternative reinforcing materials that both perform well in products, such as fluid seals and friction applications, and that are low in cost. Despite the numerous disclosures proposing lower cost alternative reinforcement materials, many of these proposed products do not adequately perform in use, cost significantly more than currently commercial products, or have other negative attributes. As such, there remains a need for reinforcement materials that exhibit high wear and heat resistance that is comparable or less expensive than other commercially available reinforcement materials.